CaMKIIδ differentially alters the passive mechanics of myocardium

Abstract

Abnormal diastolic function as seen in failing hearts with preserved ejection fraction (HFpEF) has been linked to alterations in the passive material property of myocardium. Specifically, the non‐crossbridge forming protein named titin contributes a stiffness that varies with fluctuating calcium concentration and has been thought to dominate the passive stiffness of myocardium throughout normal physiological range.Titin is expressed in two isoforms in the heart: the N2B, which has a single spring‐like element, and a more compliant isoform, N2BA, which contains two spring‐like domains (the N2B and N2A). The N2B element exists in all isoforms and is cardiac specific and consists of a large 575 residue unique sequence (N2B‐Us), three flanking immunoglobulin domains, and two other smaller unique sequences. Titin‐based stiffness of myocardium has been shown to be modulated by altered ratio expression of titin isoforms as well as post‐translational modification with unbalanced phosphorylation of cardiac titin characterizing HFpEF. Several proteins can alter titin stiffness including protein kinase (PK) G, extracellular signal‐regulated kinase 2 (ERK‐2), PKC, PKA and calcium calmodulin‐dependent protein kinase II (CaMKII) with varied effects depending on the target site location on titin.CaMKII is a dodecameric enzyme consisting of subunits encoded by four different genes known as CaMKII α,β,γ, and δ. CaMKIIδ is the predominant isoform in the heart and is alternatively spliced in the heart to generate δb and δc differing only by an 11‐amino acid nuclear localization signal. CaMKIIδ heteromultimerization is permissive, however, in that the enzyme may contain subunits from multiple CaMK genes and splice variants of those genes. Thus, it is hypothesized that the general ratio of CaMKIIδ subtypes determine its exact localization within the cardiomyocyte.CaMKIIδ knock‐out has recently been shown to ameliorate heart failure development in response to pressure overload, Gαq expression, and isoproterenol infusion while CaMKIIδ subtypes show a differential regulation of calcium handling in‐vitro and infarct formation following ex‐vivo myocardial ischemia/reperfusion. The idea that the specific CaMKIIδ subtypes differentially regulate cardiac mechanics is interesting yet remains unresolved.In our work, 5–6 weeks old mice models overexpressing CaMKIIδb and CaMKIIδc were bred against a CaMKIIδ knock‐out background, called KOB and KOC respectively. Results indicate that KOB mice show increased passive stiffness (12 mN/mm^2 +− 3 mN/mm^2) compared to KOC passive stiffness (6 mN/mm^2 +− 1 mN/mm^2) at 5–6 weeks prior to heart failure phenotype. Furthermore, as the mice progress into heart failure, the heart passive mechanics of KOB and KOC are consistent, thus, highlighting CaMKII as a precursor to heart failure development.Support or Funding InformationPresident's Disseration Year FellowshipFigure 1